A drilling system, method and drilling apparatus for determining a parameter of a formation at a downhole location is disclosed. A member having a longitudinal axis is disposed in a borehole at the downhole location. A first magnetometer and a second magnetometer are disposed at an axial location of the member as separate azimuthal locations around the member and oriented tangential to a circumference of circle in a plane transverse to a longitudinal axis of the member. Magnetic fields resulting from a current flowing longitudinally through the member due to a spontaneous potential in the formation are measured to determine the spontaneous potential. The parameter of the formation is determined from the determined spontaneous potential.